The compounds of formula I are TLR agonists. More particularly, the compounds are TLR8 agonists and may be useful for the treatment and prevention (e.g. vaccination) of cancer, autoimmune diseases, inflammation, sepsis, allergy, asthma, graft rejection, graft-versus-host disease, immunodeficiencies, and infectious diseases.
Toll-like receptors (TLRs) are a family of membrane-spanning receptors that are expressed on cells of the immune system like dendritic cells, macrophages, monocytes, T cells, B cells, NK cells and mast cells but also on a variety of non-immune cells such as endothelial cells, epithelial cells and even tumor cells (Kawai et al., Immunity, 2011, 34, 637-650, Kawai et al., Nat. Immunol., 2010, 11, 373-384). TLRs that recognize bacterial and fungal components are expressed on the cell surface (i.e. TLR1, 2, 4, 5 and 6), while others that recognize viral or microbial nucleic acids like TLR3, 7, 8 and 9 are localized to the endolysosomal/phagosomal compartment (Henessy et al. Nat. Rev. Drug Discovery 2010, 9, 293-307) and predominantly found to be expressed by cells of the myeloid lineage. TLR ligation leads to activation of NF-κB and IRF-dependent pathways with the specific activation sequence and response with respect to the specific TLR and cell type. While TLR7 is mainly expressed in all dendritic cells subtypes (DC and here highly in pDC, plasmacytoid DC) and can be induced in B cells upon IFNα stimulation (Bekeredjian-Ding et al. J. Immunology 2005, 174:4043-4050), TLR8 expression is rather restricted to monocytes, macrophages and myeloid DC. TLR8 signaling via MyD88 can be activated by bacterial single stranded RNA, small molecule agonists and lately discovered microRNAs (Chen et al. RNA 2013, 19:737-739). The activation of TLR8 results in the production of various pro-inflammatory cytokines such as IL-6, IL-12 and TNF-α as well as enhanced expression of co-stimulatory molecules, such as CD80, CD86, and chemokine receptors (Cros et al. Immunity 2010, 33:375-386). In addition, TLR8 activation can induce type I interferon (IFNβ) in primary human monocytes (Pang et al. BMC Immunology 2011, 12:55).
Small molecule agonists for both the TLR7 and TLR8 receptor as well as analogs modified for use as vaccine adjuvants or conjugates have been identified in many patents (i.e. WO1992015582, WO2007024612, WO2009111337, WO2010093436, WO2011017611, WO2011068233, WO2011139348, WO2012066336, WO2012167081, WO2013033345, WO2013166110, and US2013202629). Clinical experience has been obtained mainly for TLR7 agonists, but only very few clinical studies focused on using highly specific TLR8 agonists. To date, the only FDA (U.S. Food and Drug Administration)-approved small molecule drug is the TLR7 agonist imiquimod (ALDARA™) as a topical agent for the treatment of genital warts, superficial basal cell carcinoma and actinic keratosis. Systemic application however of the early TLR7 agonists like resiquimod has been abandoned due to intolerable cardiotoxicity observed upon global chemokine stimulation at therapeutic levels (Holldack, Drug Discovery Today, 2013, 1-4). Knowledge about TLR8 agonists is less advanced and mostly restricted to data with early mixed TLR7/8 agonists like resiquimod. For the resiquimod agonist, however, the stimulatory capacity of the TLR7 is superior compared to the activation of the TLR8, so that most of the effects of resiquimod are dominated by the effect of TLR7 activity. More recently, TLR8 specific compounds like VTX-2337 have been described by VentiRX Pharmaceuticals (i.e. WO 2007024612), allowing for the first time to analyse the specific role of TLR8 without activation of TLR7 at the same time.